Direct epoxidation of higher olefins (containing three or more carbons) such as propylene with oxygen and hydrogen has been the focus of recent efforts. For example, the reaction may be performed in the presence of a catalyst comprising gold and a titanium-containing support (see, e.g., U.S. Pat. Nos. 5,623,090, 6,362,349, and 6,646,142), or a catalyst containing palladium and a titanium zeolite (see, e.g., JP 4-352771).
Mixed catalyst systems for olefin epoxidation with hydrogen and oxygen have also been disclosed. For example, Example 13 of JP 4-352771 describes the use of a mixture of titanosilicate and Pd-on-carbon for propylene epoxidation. U.S. Pat. No. 6,008,388 describes a catalyst comprising a noble metal and a titanium or vanadium zeolite, but additionally teaches that the Pd can be incorporated into a support before mixing with the zeolite. The catalyst supports disclosed include silica, alumina, and activated carbon. U.S. Pat. No. 6,498,259 discloses the epoxidation of an olefin with hydrogen and oxygen in a solvent containing a buffer in the presence of a catalyst mixture containing a titanium zeolite and a noble metal catalyst.
In a slurry epoxidation process using the mixed catalyst systems, liquid and/or gas product streams need to be separated from the solid catalyst particles. Generally it is necessary to make titanium zeolites and the supported noble metal catalyst into large enough particles (e.g., >1 μm) to make such separation (e.g., filtration) practically viable.